Amniocentesis for prenatal diagnosis is currently utilized by 30 to 40 percent of women becoming pregnant over age 35. It's safety and accuracy are well documented. However, it can not be performed until 16 weeks gestation. Chorion villus sampling is performed between 6 and 12 weeks gestation. Although earlier prenatal diagnosis allows safer methods of pregnancy termination and reduced psychological stress, the safety and accuracy of CVS are unknown. A multicenter cooperative clinical study is proposed to specifically test two issues concerning CVS: 1) What is the risk? Specifically, is CVS associated with an increased rate of spontaneous abortion, pregnancy complications, or physical or developmental risk to the fetus? 2) Is the laboratory data from CVS accurate? The study will involve 4 to 8 clinical centers with extensive experience in amniocentesis. The centers should have experience with CVS in at least 20 ongoing pregnancies by January 1985, and the ability to contribute a minimum of 15 CVS patients per month during the study. A total enrollment of at least 2,500 subjects is desired, many of whom would accept randomization to either amniocentesis or CVS. Centers must be willing to abide by a common protocol which will be developed during the first phase. Centers should make every effort to encourage patients to participate in a randomized trial of CVS versus amniocentesis. Data from patients who refuse randomization but select CVS or amniocentesis will be analyzed separately. Patients will be counseled as to the potential risks and benefits of CVS and amniocentesis and sign and informed consent. Some patients will be automatically excluded from CVS: patients with multiple gestations, advanced gestational ages, increased risk for neural tube defects, multiple miscarriages, or positive gonorrhea cultures. Amniocentesis will be performed by established methods at 16-17 weeks. CVS will be performed at 7-11 weeks using a catheter-aspiration technique under real time ultrasound guidance. Folow-up ultrasound and maternal serum AFP study will be performed at 18 weeks. In the cases of failure to obtain diagnoses or ambiguous diagnoses by CVS, follow-up amniocentesis will be recommended. Pathologic exam and laboratory confirmation studies will be performed on all spontaneous abortions, elective abortions, and stillborns. A standard follow-up procedure will be utilized for all newborns, and pediatric exam on all infants will be performed at 1 year of age.